Within the past decade, the advent of world-wide electronic communications systems has enhanced the way in which people can send and receive information. In particular, the capabilities of real-time video and audio systems have greatly improved in recent years. In order to provide services such as video-on-demand and video conferencing to subscribers, an enormous amount of network bandwidth is required. In fact, network bandwidth is often the main inhibitor in the effectiveness of such systems.
In order to overcome the constraints imposed by networks, compression systems have emerged. These systems reduce the amount of video and audio data which must be transmitted by removing redundancy in the picture sequence. At the receiving end, the picture sequence is uncompressed and may be displayed in real-time.
One example of a video compression standard is the Moving Picture Experts Group (“MPEG”) standard. Within the MPEG standard, video compression is defined both within a given picture and between pictures. Video compression within a picture is accomplished by conversion of the digital image from the time domain to the frequency domain by a discrete cosine transform, quantization, and variable length coding. Video compression between pictures is accomplished via a process referred to as motion estimation and compensation, in which a motion vector plus difference data is used to describe the translation of a set of picture elements (pels) from one picture to another.
The ISO MPEG-2 standard specifies only the syntax of bitstream and semantics of the decoding process. The choice of coding parameters and trade-offs in performance versus complexity are left to the encoder developers.
One aspect of the encoding process is compressing a digital video image into as small a bitstream as possible while still maintaining video detail and quality. The MPEG standard places limitations on the size of the bitstream, and requires that the encoder be able to perform the encoding process. Thus, simply optimizing the bit rate to maintain desired picture quality and detail can be difficult.
The MPEG-2 standard is designed for motion video. Many coding tools and options are defined in the standard to achieve high quality pictures at low bit rates. One significant feature of video compression in MPEG-2 is adaptive quantization, meaning that the quantization level is adjustable from one picture to the next and from one macroblock to the next within a picture. This flexibility allows an encoder to balance the output bitstream size and thereby achieve a constant bit rate output. Variation in quantization level also allows each compressed picture to have a different amount of encode bits based on complexity of intra and inter-picture characteristics.
When the input video stream is constant, i.e., one picture appears to be the exact replica of the previous picture and an exact replica of the next picture, this is called a series of still frames. Slight variations in the amount of detail of an encoded and then decoded macroblock of a current frame compared to the same encoded and then decoded macroblock of a prior frame or a next frame in the series of still frames can create fluctuation in luminance and/or chrominance data which can appear as movement between the frames notwithstanding that the frames actually comprise a series of still frames. This appearance of movement is referred to as pulsation artifacts. For instance, variation in chrominance data from a prior frame to a current frame can create differences in shade of the same color. These differences in color shade effectively create pulsation artifacts which can cause a series of still pictures to come alive and no longer resemble the original input video.
This invention thus seeks to enhance picture quality of an encoded video sequence having a series of still frames or partially still frames to enhance the encoding of the frames and prevent pulsation artifacts from, for example, still frame to still frame.